With ever-increasing electrical power requirements in motor vehicles, dual voltage automotive electrical systems have been proposed as a means of improving the system operating efficiency. Typically, the alternator and storage battery operate at a relatively high voltage level (such as 42 volts) which is supplied to most of the high power electrical loads, and a DC-DC converter is used to supply power at a lower voltage (such as 14 volts) to certain loads such as lamps, small motors, and various electronic controllers. FIG. 1 depicts a dual voltage automotive electrical system 10, in which an engine or motor driven 42-volt alternator 11 is connected across storage battery 12 having a nominal terminal voltage of 36-volts. A voltage regulator 14 responsive to the upper system voltage on line 15 controls the excitation of alternator 11 so as to regulate the upper system voltage at the nominal 42-volts. The alternator 11 and/or storage battery 12 provide power directly to the majority of the electrical loads of the vehicle, designated as the 42V Loads 19. Various other loads, designated as the 14V Loads 20, are powered by the DC-DC converter 18, which converts the 42-volt input of alternator 11 and/or storage battery 12 to a nominal 14-volt output. The performance requirements of the converter 18 include: low cost, high reliability, long life at high operating temperatures, high power density, light weight, low conducted and radiated emissions (electromagnetic compatibility), and protection against overload and reverse polarity.
The most common approaches in converter design for the dual-voltage automotive application are illustrated in FIGS. 2 and 3, where the converter 18 is designated respectively by the reference numerals 18a and 18b. In each case, the input voltage is depicted as 42-volts, and the converter supplies power at an output voltage of 14-volts. In the approach of FIG. 2, a power-FET Q1 connected between input capacitor C1 and inductor L1 is pulse-width-modulated to charge an output capacitor C2 and deliver power to the 14V Loads 19 through the inductor L1. During off periods of the FET Q1, energy stored in inductor L1 is circulated through output capacitor C2 and the 14V Loads 19 via free-wheeling diode D1. If desired, the diode D1 can be replaced with another FET controlled in relation to the conduction of FET Q1 to provide synchronous rectification. In FIG. 3, the converter 18b transfers power through a center-tapped step-down transformer T1 to provide isolation between the input and output; in this case, the FETs Q1 and Q2 are pulse-width-modulated to excite the transformer primary windings P1 and P2, with the diodes D2 and D3 rectifying the voltage appearing across the transformer secondary windings S1 and S2. The inductor L1, the capacitors C1 and C2, and the free-wheeling diode D1 are identical to those shown in the converter of FIG. 2.
The converters of FIGS. 2 and 3 are characterized by high switching losses, particularly at higher operating frequencies. In general, these losses limit the operating frequency to about 100 kHz or lower, especially at power levels above about several hundred watts. Additionally, there is no inherent overload protection, and the hard switching produces unacceptably high electromagnetic interference without substantial filtering. As a result, it is difficult to meet the objectives of low cost, high reliability and long life at high operating temperatures.